Device and method for sheathing a ply of threads

ABSTRACT

A device for continuously sheathing a ply of threads, said ply being formed by an array of approximately mutually parallel threads (F). The device comprises a thread guide ( 35 ), a coating chamber ( 31 ) into which a first feed channel ( 10 ) and a second feed channel ( 20 ) run, which are independent of each other, connected to a first feed means ( 13 ) and to a second feed means ( 23 ) respectively and capable of delivering a first material (A) and a second material (B) under pressure and with a defined flow rate, and the outlets ( 11, 21 ) of which channels are placed above and below the plane of the ply of threads (F), and an output die. Pressure-measuring means ( 12, 22 ), connected to a controller for controlling the pressure of each of the feed means ( 13, 23 ), are placed in the coating chamber ( 31 ) facing and in line with each other, on either side of the plane of the ply and in the immediate vicinity of the outlet for the feed channels ( 11, 21 ).

RELATED APPLICATIONS

This application claims the priority of French application no. 07/03926filed May 31, 2007, the content of which is hereby incorporated byreference.

FIELD OF THE INVENTION

The invention relates to the field of the manufacture of compositereinforcement products made up of threads embedded in thermoplastic orviscoelastic material.

More particularly, the invention relates to the manufacture ofreinforcement plies widely used in the tires with which terrestrialvehicles are intended to be equipped.

BACKGROUND OF THE INVENTION

These plies, which are intended to constitute the carcase reinforcementor crown reinforcement plies of the tire, are formed from continuousthreads that are approximately mutually parallel and arranged with agiven transverse pitch. The threads are embedded in rubber.

The term “thread” is understood to mean any individual or cabledreinforcement thread, which is itself formed by an assembly ofindividual filaments. The threads may be, without distinction, oftextile, composite or metallic nature.

The processes most widely used to manufacture these straight-threadplies consist in depositing a layer of rubber on both sides of the plyof threads using a calendar formed from two rolls capable of pressingthe rubber sheets against each other so that the rubber penetrates intothe free space between the threads until forming rubber bridges.

Also known are sheathing means in which the ply of threads passesthrough a coating die upstream of which there is a coating chambercommunicating with a feed means capable of delivering a rubbery productunder pressure and at a given flow rate. The ply of threads runs from athread guide placed at the inlet of the device into the coating chamber,where the mixture under pressure penetrates into the free spaces betweenthe threads, and emerges from the device by passing through a sizing dieintended to give the semi-finished ply a precise thickness.

Material feed means, such as for example one or more single-screwextruders, feed the coating chamber via one or more channels openingonto the upper part or onto the lower part of the ply of threads.

This technique has, inter alia, the advantage of allowing very regularseries of rubber bridges to be formed because of the very goodimpregnation of the threads due to the high pressure within the coatingchamber. Its use is particularly suitable for producing narrowstraight-thread plies, similar to strips, and intended for production,by winding said strips onto the blank of a tyre in the process of beingbuilt. Another application consists in producing reinforcement pliesmaking a given angle to the circumferential direction, by juxtaposingportions of strips at a given angle.

However, the manufacture of modern tyres requires assemblies havingincreasingly complex and precise geometries, making it necessary toreconsider the use of the known means.

FIG. 1 shows a straight-thread ply of the type sought, in which therubber compound A covers the threads F so that the upper back of thethreads is located at a distance e_(up) from the upper surface of theply and the lower back is located at a distance e_(low) from the lowersurface of the ply, the total thickness of which is denoted by E_(tot).It should also be noted that, in this type of ply, the distances e_(up)and e_(low) are not necessarily the same.

To manufacture a straight-thread ply as illustrated in FIG. 1 using asheathing device, the thicknesses e_(up) and e_(low) are varied bydisplacing the plane of the ply of threads relative to the upper andlower edges of the die. The total thickness e_(tot) is determined by thediameter of the threads, the run speed of the threads through thesheathing device and the pressure within the coating chamber.

This results in a number of difficulties associated with controlling theposition of the ply of threads relative to the surface. This is becauseany variation in pressure between the flow of compound intended to feedthe upper part of the ply and the flow of compound intended to feed thelower part of the ply causes the threads to move upwards or downwards.

To give an illustration, a pressure difference of 1 bar between the twoflows over a length of 10 mm generates a force of 0.6 N perpendicular tothe surface of the ply. It would be possible to increase the tension ofthe ply so as to prevent them from undergoing too great a displacementin the direction perpendicular to the surface of the ply. However, thistechnique remains limited in that the threads would then be liable tocut or damage the ply formed by an uncured and still very plasticelastomer.

The differences in pressure between the flows intended to feed materialinto the coating chamber, at the upper part and the lower part of theply of threads respectively, are the consequence of a large number ofparameters that it is not easy to bring under control. This applies inparticular when it is desired to vary the pressure in the coatingchamber according to the run speed of the ply of threads so as tomaintain a constant total thickness E_(tot). Other parameters also havean influence, such as the instantaneous temperature of the members incontact with the rubber compound, the localized differences in rheologyof the compound, the extrusion speed, the diameter of the cords, thepitch of the cords and finally the shape and the geometry of the flowchannels.

This results in a geometric irregularity in the rubber thicknesses onthe back of the threads, which is not compatible with the desiredprecision.

SUMMARY OF THE INVENTION

One object of the invention is to provide a solution to the statedproblem and to allow straight-thread plies to be manufactured bysheathing with the desired precision.

This and other objects are attained in accordance with one aspect of thepresent invention directed to a sheathing device for continuouslysheathing a ply of threads with a thermoplastic or viscoelastic material(A, B), said ply being formed by an array of approximately mutuallyparallel threads (F), which device comprises a thread guide (35); acoating chamber (31) into which a first feed channel (10) and a secondfeed channel (20) run, which are independent of each other, connected toa first feed means (13) and to a second feed means (23) respectively andcapable of delivering a first material (A) and a second material (B)under pressure and with a defined flow rate, and the outlets (11, 21) ofwhich channels are placed above and below the plane of the ply ofthreads (F); an output die (32), through which guide, chamber and diethe ply of threads (F) runs when the device is in operation; andpressure-measuring means (12, 22), coupled to a controller (40) forcontrolling the pressure of each of the feed means (13, 23), and locatedin the coating chamber (31) facing and in line with each other, oneither side of the plane of the ply and in the immediate vicinity of theoutlet (11, 21) for the feed channels (10, 20).

It is, therefore, possible to regulate said feed means in such a waythat the pressure of the compound output by the first feed channel isequal to the pressure of the compound output by the second feed channel,irrespective of the pressure level that it is desired to establish insaid chamber in order to regulate the total thickness E_(tot) of thecoated ply as a function of the speed of the ply of threads.

The pressure difference between the upper part and the lower part of theply of threads is always substantially equal to zero, thereby preventingany displacement of the plane of the threads relative to the outlet die.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic sectional view of a ply of coated threadsaccording to a first embodiment, and obtained with the process accordingto an embodiment of the invention;

FIG. 2 shows a schematic sectional view of a device in accordance withan embodiment of the invention; and

FIG. 3 shows a schematic sectional view of a ply of threads coatedaccording to a second embodiment of the invention, and obtained by aprocess according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

As mentioned in the above paragraphs, FIG. 1 shows a first embodiment ofa ply of coated threads that can be obtained with the process and thedevice according to an embodiment of the invention.

FIG. 2 illustrates a device according to an embodiment of the invention.The sheathing means is formed by a thread guide 35 located at the inletof the device, and a coating chamber 31 into which a first feed channel10 runs, the outlet 11 of which channel is placed above the plane of theply of threads F. The outlet 21 of a second feed channel 20 also runsinto the coating chamber 31 below the plane of the ply of threads F. Theoutlet of the device is formed by the die 32, which comprises an upperedge 33 and a lower edge 34, the separation between which serves toregulate the total thickness E_(tot) of the coated ply. Means (notshown) cause the ply of threads to run at a given speed through thesheathing device. These means also place the threads F under tension.

Feed means 13 and 23 are connected to the feed channels 10 and 20respectively. These feed means may be conventional extrusion means, ofthe type of those widely used in the rubber industry and comprising ascrew rotated by a motor (not shown). Equivalently, and when the coatingmaterial is more fluid, it is possible to use a rotary pump or apositive-displacement pump. These means are capable each in their ownway, to deliver the material (A, B) intended to coat the threads with agiven flow rate and at a given pressure into the coating chamber (31)via the feed channels 10 and 20.

The material feed means 13 and 23 are separate from each other.

The device includes pressure-determining means 12 and 22, such aspressure sensors, placed in the coating chamber, facing and in line witheach other, on either side of the plane of the ply, so as to measure thepressure at the point where the compound is liable to influence theposition of the threads.

These pressure sensors are placed as close as possible to the outlets 11and 21 of the feed channels 10 and 11 in such a way that the pressuredifference between the two sides of the ply is dependent both on thepressure delivered by the feed means running into the outlet inquestion, and pressure variations occurring at the moment of coating thethreads with each of the compounds and capable of causing displacementof the threads.

It is important for the sensors to be located quite precisely in linewith each other so as to measure the forces that are exertedperpendicular to the plane of the ply at a given point.

These pressure-determining means are connected to a controller 40 forcontrolling the feed means 13, 23, so as to regulate the pressure ineach of the feed channels according to a given setpoint.

Arrangements are therefore made for the pressure difference on eitherside of the plane of the ply to be approximately zero.

To prevent a slow drift of the system, the controlling of one of thefeed means serves as a master means to which the controlling of theother feed means is slaved.

This device is therefore distinguished from the known regulating means,in which the pressure measurement means are placed inside the feedchannel, and which do not allow fine changes in the pressure differencesin a direction orthogonal to the plane of the ply to be detected.

FIG. 2 also illustrates an alternative embodiment of the device, inwhich the outlet 11 of the channel 10 is offset by a distance d relativeto the outlet 21 of the channel 20 along the direction in which the plyof threads advances through the device.

This arrangement has the effect that the ply of threads is still partlyheld in place by the thread guide when the compound penetrates the upperpart of the ply of threads. The threads are still supported on thethread guide, which prevents any movement of the ply of threads in adirection perpendicular to the plane of the ply.

The ply of threads is completely free of the influence of the threadguide only at the outlet 21 of the other feed means so that, when thecompound penetrates into the lower part of the ply of threads, said plyis supported on the compound of the upper part. This results in betterstability of the position of the threads and consequently betterprecision of the thicknesses e_(up) and e_(low).

As a result, the pressure sensors are placed close to the second end 21and offset by a distance greater than d relative to the first outlet 11.

The device also has another advantage over the conventional devices inwhich the compound is from a single feed means.

This is because it is often necessary to produce a ply in which therubber compound A forming the upper part of the ply is different fromthe rubber compound B forming the lower part of the ply, as illustratedby FIG. 3. In this configuration, the position of the centre of thethreads, depicted in FIG. 3 by a dotted line, does not necessarilycorrespond to the line of separation between the compound A and thecompound B.

It is therefore necessary for the respective thicknesses a and b of thecompounds A and B to be controlled more precisely, the sum of saidthicknesses being equal to the total thickness E_(tot).

It has been demonstrated that it is possible to vary the thickness arelative to the thickness b by varying the distance d separating theoutlet 11 of the feed channel 10 from the outlet 21 of the feed channel20 without it being necessary to vary the pressure in the coatingchamber, this being of primary importance for controlling the totalthickness, as mentioned in the above paragraphs.

Another use of the device according to the invention consists inrecording the pressure difference between the two measurement means 12and 22 for fine control of the end product leaving the device. It isthus possible, when this difference exceeds a certain threshold, toconclude that the zone in which this anomaly occurs is unsuitable forbeing used in a subsequent manufacturing step. It suffices therefore toidentify the zone in question by any means, without it being necessaryto interrupt production of the coated ply, and to eliminate it from asubsequent manufacturing phase.

The device forming the subject of the present invention therefore makesit possible to solve the stated problem, which relates to controllingthe geometry of the reinforcement plies, and also offers many prospectsof varying the construction of tires.

1. A sheathing device for continuously sheathing a ply of threads with athermoplastic or viscoelastic material (A, B), said ply being formed byan array of approximately mutually parallel threads (F), which devicecomprises: a thread guide (35); a coating chamber (31) into which afirst feed channel (10) and a second feed channel (20) run, which areindependent of each other, connected to a first feed means (13) and to asecond feed means (23) respectively and capable of delivering a firstmaterial (A) and a second material (B) under pressure and with a definedflow rate, and the outlets (11, 21) of which channels are placed aboveand below the plane of the ply of threads (F); an output die (32),through which guide, chamber and die the ply of threads (F) runs whenthe device is in operation; and pressure-measuring means (12, 22),coupled to a controller (40) for controlling the pressure of each of thefeed means (13, 23), and located in the coating chamber (31) facing andin line with each other, on either side of the plane of the ply and inthe immediate vicinity of the outlet (11, 21) for the feed channels (10,20).
 2. The sheathing device according to claim 1, in which the outlets(11, 21) of the feed channels are offset by a given distance (d) alongthe direction in which the ply of threads according to the inventionadvances.
 3. A sheathing method for continuously sheathing a ply ofthreads with a thermoplastic or viscoelastic material (A, B), said plybeing formed by an array of approximately mutually parallel threads (F)running in a given direction through a sheathing device according toclaim 1, wherein the operation of the feed means (13, 23) is regulatedin such a way that the pressure difference measured by the twopressure-measurement means located in the coating chamber (31) is zero.4. The sheathing method according to claim 3, in which the materials (A,B) are rubber-based viscoelastic materials.
 5. The sheathing methodaccording to claim 3, in which the material (A) deposited on the upperface is different from the material (B) deposited on the lower face. 6.The sheathing method according to claim 5, in which the distance (d)between the outlets (11, 21) of the feed channels (10, 11) is varied soas to cause the thickness (a) of material (A) deposited on the upperface of the ply of threads relative to the thickness (b) of material (B)deposited on the lower face of the ply of threads to vary.
 7. Thesheathing method according to claim 3, in which the calculation of thepressure difference between the two pressure measurement means (12, 22)located in the coating chamber (31) is used to determine the conformityof the final product obtained.